I beleive what you want to do is connect the LAN port from m0n0wall
into a switch port for WRT54G. Disable DHCP in the WRT54G as pc's
connected via a switch port AND wireless connections will get DHCP
assignments from M0n0wall. Basically use the WRT54G an AP, not a
router. Do not connect anything in the WAN port of the WRT54G.
I have a similiar setup, but am using two WRT54G, with WDS connecting
the two. I am planning on replacing the first WRT54 in place of a
m0n0wall appliance. The second m0n0wall will still provides wireless
access.
I am using OpenWRT in each WRT54GS router in my house.
~Don